1. Field of the Invention
The present invention concerns the thermal protection of structural surfaces in the presence of an erosive flow. The invention is more particularly concerned with a thermal protection device and a method of fabricating the latter, that is intended to protect structural surfaces exposed to an erosive, and possibly corrosive, flow of gas at high speed and high temperature and to high levels of vibration, in particular when this is inherent to their operation. It is to be understood that the high-speed flow is a relative flow in the sense that it can be the movement of a vehicle in a gaseous atmosphere or a high-speed gaseous flow relative to a fixed structure, for example in a propulsion nozzle.
2. Description of the Related Art
A routine way to protect a surface from such flow is to use ablative (or ablatable) thermal protection, i.e. a coating that protects the surface and is consumed. This type of protection naturally applies only to short term exposure (in practice a few hundred seconds maximum).
The thermal protection layers of structures to be protected from an erosive flow are usually made up of composite materials with organic, organo-metallic or mineral binders capable of including powder, fiber, organic or mineral woven reinforcements. Due to the action of the hot gases, the ablatable material is subjected to the phenomenon of pyrolysis. This pyrolysis is accompanied by degradation of the carbon-containing or organo-silicic chain of the binder that renders the slag fragile and liable to break. To overcome this the thermal protection layer is routinely reinforced using various techniques.
The abrasion of an ablatable material of this type exposed to erosion by hot gases and to vibration is known to be reduced if the material is reinforced. The reinforcement embedded in the insulative material can be a metal or non-metal, woven or fibrous. There is less ablation if the fibrous or woven reinforcement is anchored and oriented in the matrix perpendicularly to the direction of the gases and there is less heat transfer if the reinforcement is oriented in the direction of the gases. The fibrous or woven reinforcements appear to offer less thermal insulation than the insulative matrix.
There are two main types of methods of making thermal protection materials:
1) a first type of method uses compression at very high pressure (typically several hundred bars) of a mass of reinforcing fibers pre-impregnated with resin; this is a kind of molding with injection of the pre-impregnated fiber mass between two half-shells; and
2) a second type of method involves winding on a fiber impregnated with a resin, impregnation preferably being effected during the winding operation.
The winding method has the advantage of not requiring such high pressures as are required by the compression method; the winding method can include a pressurized step, but the pressure in question is typically in the order of approximately ten bars at most.
Examples of thermal protection devices are given by the documents EP-0.174.886, EP-0.398.787, FR-2.652.036, EP-0.471.605 and EP-0.501.861, covering prior inventions made by the Applicant.
Document EP-0.174.886 discusses thermal protection including an insulative polymerized resin layer fixed to the surface of the wall to be protected; this layer includes an armature having a fringed mesh with a mesh part exposed to the erosive flow and fringes directed, with a predetermined inclination, towards the surface to be protected. Generally speaking, this disclosure concerns the protection of hollow structures with a monotonously changing section.
Related art document EP-0.398.787 proposes an improved solution to the above disclosure in the sense that it teaches thermal protection including, as in the previous solution, a refractory armature formed of a fringed mattress with a mesh part exposed to the erosive flow and fringes adapted to be directed towards the structure wall to be protected. The armature is embedded in a thermally insulative matrix. This thermal protection further includes a wide-mesh woven refractory material disposed parallel to the mesh part of the armature with the fringes passing through the wide mesh. The document describes a protection layer, advantageously a refractory protection layer, facing the ends of the fringes and which in practice includes a wound filament or tape binding the woven refractory material. This layer is advantageously eliminated after curing, during final machining of the thermal protection, so that in practice none of it remains during use of the thermal protection.
Related art document FR-2.652.036 proposes a thermal protection coating having a different structure in that it includes a main layer formed of a succession of refractory fibrous reinforcement slices substantially parallel to each other but inclined to the surface to be protected and between which insulative slices are interleaved, this main layer being lined with at least one sub-layer extending along the surface to be protected and essentially consisting of an insulative material compatible with that of the insulative slices. The refractory fibrous reinforcement is in practice a tape. The sub-layer is formed of the same insulative material as the insulative slices, for example. This sub-layer serves mainly to anchor the fibrous refractory reinforcement since, during the fabrication of the thermal protection coating, the latter is engaged in grooves formed in this sub-layer. The latter can also have other functions, such as protection against X-rays or super-insulation using cellular material.
A third type of thermal protection is proposed in document EP-0.471.605 wherein thermal protection is obtained by winding a plurality of superposed layers of plush refractory filaments around a mandrel temporarily provided with radial barbs and reinforcing pins, also made of refractory material, that form an integral part of the finished thermal protection.
Finally, document EP-0.501.861 proposes three-dimensional thermal protection as in the previous document formed of a stack of impregnated woven materials traversed by refractory material fibers.
These various solutions constitute undoubted progress. Nevertheless, there is a need for non-ablative thermal protection resistant to an erosive flow of gas at high speed and to severe thermal aggression, either for longer time periods or for the same time periods, for parts where the geometry must remain unchanged.
An object of the invention is to meet this need.
To this end the invention proposes a thermal protection device adapted to extend along a surface to be protected from a thermally and mechanically severe external environment, having an inside face adapted to face the surface to be protected and an outside face adapted to be exposed to the external environment, including a composite layer containing an armature buried in a thermally insulative material matrix. The thermal protection device further includes a ceramic, metal or metalloid external layer of which the outside face is part and that is attached to the composite layer.
Accordingly, in accordance with the invention, the external layer can be a layer of ceramic (for example alumina, titanium dioxide, possibly combined with alumina, zirconia, zircon (ZrO2+SiO2), spinel (MgO, Al2O3), etc., alone or in combination) or of metal (including molybdenum, tungsten, titanium, etc., alone or in combination) and/or of a metalloid (including titanium carbon, tungsten carbide, etc., alone or in combination). Generally speaking, these elements can be single or combined (metals, oxides, carbides, nitrides, borides, etc., alone or in combination), having a high melting point, typically greater than 1,600xc2x0 C., possibly approximately 2,000xc2x0 C., even 2,500xc2x0 C.
It has previously been proposed to associate a second layer with the composite layer but the latter has generally been disposed between the composite layer and the surface to be protected. On the other hand, the invention teaches the provision of the composite layer with an external layer directly exposed to the aggressive external environment. This approach goes somewhat against the teachings of the known prior art since the prior art documents referred to above would lead a person skilled in the art to consider it necessary, in order to achieve good retention of the exterior part of the thermal protection, to provide an anchorage in the direction of the thickness of the composite layer, in practice by means of fringes or tapes. However, and very surprisingly, it has been found that a clear improvement in resistance to a thermally and mechanically aggressive environment can be obtained, without using such transverse reinforcements, by the simple addition to the composite layer of a ceramic, metal or metalloid external layer.
In accordance with preferred features of the invention:
the device includes an intermediate bonding layer between the composite layer and the ceramic, metal or metalloid external layer;
the refractory armature includes a mesh part and fringes attached to the mesh part;
the mesh part extends along the inside surface and the fringes extend at least partly towards the external layer;
the fringes have a non-zero inclination less than 90xc2x0 to the inside and outside faces, preferably less than 45xc2x0, for example between 20xc2x0 and 40xc2x0;
the thermal protection device has an axis of revolution;
the refractory armature is of silica and the matrix is of phenolic resin;
the external layer can be of ceramic, for example formed of one or more oxides such as alumina, spinel (MgO, A12O3), zirconia, possibly combined with silica (zircon), titanium dioxide, etc., alone or in combination; it can in particular be alumina, alone or in combination with titanium dioxide (preferably in a ratio of 60/40 percent by weight), or even spinel, or zirconia;
the external layer can also be a metal such as molybdenum, tungsten, or titanium;
the external layer can also be a metalloid, such as carbides of titanium and/or tungsten in particular;
the external layer has a melting point greater than approximately 1,600xc2x0 C., preferably greater than approximately 2,000xc2x0 C.; and
the external layer is bonded to the composite layer by a layer of copper.
The invention also proposes a method of fabricating thermal protection adapted to extend along a surface to be protected from a thermally aggressive external environment, including the following steps:
a composite layer is made containing a refractory armature buried in a thermally insulative matrix, the composite layer having a face adapted to face the surface to be protected; and
a ceramic, metal or metalloid external layer is applied at least indirectly to the composite layer, defining an external face adapted to be exposed to the thermally aggressive external environment.
According to other preferred features of the invention:
the composite layer is made by impregnating a reinforcement with a resin, by applying the impregnated reinforcement to a surface and then polymerizing the resin;
the impregnated reinforcement is applied by winding it onto a surface of revolution;
the impregnated reinforcement is applied by winding it onto the surface to be protected;
the reinforcement is a fringed mesh;
the external layer is fixed to the composite layer so that the fringes are directed towards the external layer;
the composite layer is made by imparting to the fringes a non-zero inclination less than 90xc2x0, preferably less than 45xc2x0, for example between 20xc2x0 and 40xc2x0;
the composite layer is made by impregnating a silica reinforcement with phenolic resin and then polymerizing the resin;
the external layer is made from a ceramic based on alumina;
the external layer is made from a mixture of alumina and titanium dioxide;
the mixture is prepared in a proportion of 60% alumina and 40% titanium dioxide;
the external layer is made from molybdenum;
a layer of copper is applied to the composite layer before applying the external layer; and
the external layer is applied by means of a plasma torch.
Objects, features and advantages of the invention emerge from the following description given by way of non-limiting example with reference to the appended drawings.